Proliferating cells require coordinated gene expression between the nucleus and mitochondria in order to divide, ensuring sufficient organelle number in daughter cells [1]. However, the machinery and mechanisms whereby proliferating cells monitor mitochondria and coordinate organelle biosynthesis remain poorly understood. Antibiotics inhibiting mitochondrial translation have emerged as therapeutics for human cancers because they block cell proliferation [2, 3]. These proliferative defects were attributable to modest decreases in mitochondrial respiration [3, 4], even though tumors are mainly glycolytic [5] and mitochondrial respiratory chain function appears to play a minor role in cell proliferation in vivo [6]. Here we challenge this interpretation by demonstrating that one class of antiproliferative antibiotic induces stalled mitochondrial ribosomes, which triggers a mitochondrial ribosome and RNA decay pathway. Rescue of the stalled mitochondrial ribosomes initiates a retrograde signaling response to block cell proliferation and occurs prior to any loss of mitochondrial respiration. The loss of respiratory chain function is simply a downstream effect of impaired mitochondrial translation and not the antiproliferative signal. This mitochondrial ribosome quality-control pathway is actively monitored in cells and constitutes an important organelle checkpoint for cell division.

Impaired turnover of newly synthesized mitochondrial proteins of the oxidative phosphorylation complexes leads to protein over-accumulation in the inner mitochondrial membrane, thereby generating a stress that dissipates the mitochondrial membrane potential and therefore compromises organelle and cellular fitness.

These findings indicate that mitochondrial ribosomal subunit defects can generate tissue-specific manifestations, such as cardiomyopathy.

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Mammalian mitochondrial DNA (mtDNA) is a high-copy maternally inherited genome essential for aerobic energy metabolism. Mutations in mtDNA can lead to heteroplasmy, the co-occurence of two different mtDNA variants in the same cell, which can segregate in a tissue-specific manner affecting the onset and severity of mitochondrial dysfunction. To investigate mechanisms regulating mtDNA segregation we use a heteroplasmic mouse model with two polymorphic neutral mtDNA haplotypes (NZB and BALB) that displays tissue-specific and age-dependent selection for mtDNA haplotypes. In the hematopoietic compartment there is selection for the BALB mtDNA haplotype, a phenotype that can be modified by allelic variants of Gimap3. Gimap3 is a tail-anchored member of the GTPase of the immunity-associated protein (Gimap) family of protein scaffolds important for leukocyte development and survival. Here we show how the expression of two murine Gimap3 alleles from Mus musculus domesticus and M. m. castaneus differentially affect mtDNA segregation. The castaneus allele has incorporated a uORF (upstream open reading frame) in-frame with the Gimap3 mRNA that impairs translation and imparts a negative effect on the steady-state protein abundance. We found that quantitative changes in the expression of Gimap3 and the paralogue Gimap5, which encodes a lysosomal protein, affect mtDNA segregation in the mouse hematopoietic tissues. We also show that Gimap3 localizes to the endoplasmic reticulum and not mitochondria as previously reported. Collectively these data show that the abundance of protein scaffolds on the endoplasmic reticulum and lysosomes are important to the segregation of the mitochondrial genome in the mouse hematopoietic compartment. KEYWORDS mitochondria; mitochondrial DNA; mice; segregation; Gimap M AMMALIAN mitochondrial DNA (mtDNA) is a maternally inherited small circular multicopy genome that encodes 13 proteins that are essential subunits of four of the five complexes required for mitochondrial oxidative phosphorylation. Germline or somatic-cell mtDNA mutations lead to the co-occurrence of two or more sequence variants in a cell, a state known as heteroplasmy. In the absence of selection, the segregation of mtDNA sequence variants is neutral and can be modeled as a random walk (Chinnery and Samuels 1999); however, in some cases there is preferential selection for a mtDNA sequence variant that is dependent upon the nucleotide sequence, tissue, and nuclear background (Battersby and Shoubridge 2001;Battersby et al. 2003Battersby et al. , 2005Jokinen and Battersby 2013;Burgstaller et al. 2014). The majority of pathogenic mtDNA mutations are heteroplasmic and some mutations display skewed segregation patterns in somatic tissues. (Larsson et al. 1990;Boulet et al. 1992;Kawakami et al. 1994;Dunbar et al. 1995;Fu et al. 1996;Chinnery et al. 1997Weber et al. 1997), which can affect the onset and severity of mitochondrial dysfunction. Currently, the molecular basis for this regulation of the mitochondrial genome is largely un...